An estimated 11.5 - 12 million foreign nationals are living illegally in the US, according to the Pew Hispanic Center.

These are mainly people who have entered the US without the necessary documents or who have overstayed temporary visas.

The pace of unauthorised arrivals has accelerated over the last two decades. On average, nearly five times as many illegal immigrants enter the country each year now as did in the 1980s. About 40% of illegal immigrants have been in the US five years or less.